There has heretofore been known an absorbent article such as a sanitary napkin which is provided with an adhesive part formed on a back surface of the strip-shaped and is used by having the adhesive part attached to the crotch portion of underwear.
Japanese Patent Application Publication No. Hei 11-99179 describes the following sanitary napkin as an example of such an absorbent article. Specifically, in the sanitary napkin, an elastic flap (elastic member) stretchable in the front and rear direction is provided so as to protrude from each of front and rear end portions of the napkin, and an adhesive part to be attached to underwear is formed on the back of a tip portion of the elastic flap.    Patent Document 1: Japanese Patent Application Publication No. Hei 11-99179
However, in the absorbent article as described above, when the elastic flap extended from the rear end portion is positioned near the cleavage of the buttocks, the flap deforms along the cleavage into a cross-sectionally triangular shape and the adhesive part formed on the flap also deforms into a cross-sectionally triangular shape.
Meanwhile, underwear that is not particularly very tight, such as shorts for everyday wear, deforms into a cross-sectionally triangular shape together with the elastic flaps immediately after the underwear is fitted to the body, but tends to return to its original flat shape when the wearer moves her body.
As a result, the adhesive parts are peeled off from the underwear near the cleavage and thus the elastic flaps come off from the underwear. Consequently, leakage of body fluid such as leakage of menstrual blood, for example, may occur due to a gap between the napkin and the body (particularly, the buttocks) or slippage of the napkin. Alternatively, when adhesion between the elastic flaps and the underwear is strong, the flaps may move away from the buttocks and the body fluid may leak from a gap therebetween.